1. Field of the Invention
The present invention relates to a phosphor-containing wavelength conversion sheet, and relates particularly to a wavelength conversion sheet which can convert the wavelength of a blue light or ultraviolet light from an LED by placing the sheet on the chip surface of the LED element and then curing the sheet to cover the element, a method of producing a light emitting device that uses the sheet, and a light emitting device.
2. Description of the Prior Art
In the field of light emitting diodes (LED), the use of phosphors for the purpose of wavelength conversion is already known (Patent Document 1). Because silicone resins exhibit excellent light resistance properties, they are attracting much attention as coating materials for encapsulating and protecting LED elements (Patent Document 2). Further, silicone resin sheets containing low concentrations of phosphors have also been disclosed (Patent Document 3).
Generally, in a white LED, a phosphor is dispersed in the vicinity of the LED chip by using a method in which the chip is coated with a silicone resin or epoxy resin containing a dispersed phosphor, thereby converting the blue light to a pseudo white light. However, if the dispersion of the phosphor within the resin layer lacks uniformity or is biased, then a color shift is likely to occur, and therefore in order to produce a uniform white light, it is necessary to ensure that the phosphor is dispersed uniformly within the coating resin layer. As a result, screen printing methods and methods of uniformly dispersing the phosphor near the lower region of the coating resin layer by precipitating the phosphor have been investigated, but these methods have suffered from problems, including a complex production process and an unsatisfactory level of stability. Accordingly, a technique of easily dispersing a phosphor uniformly in the vicinity of a chip surface has been keenly sought. Wavelength conversion sheets have been implemented as a measure for addressing this issue, but conventional wavelength conversion sheets contain only small amounts of phosphor, and therefore the conversion efficiency has been inadequate.
Further, in LEDs and the like, the resin layer covering the LED element also requires high levels of heat resistance and ultraviolet resistance and the like. It would be very convenient if such resin layers could be formed using conventional production apparatus.
Patent Document 1: JP 2005-524737 A
Patent Document 2: JP 2010-202801 A
Patent Document 3: JP 2009-235368 A